Saw something in a post today and it made me have to ask, what is the difference between the heads with the pop-up pistons vs the older flattop pistons... Is the heads different between the 2 ??? any pics would be great

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No pictures, but the heads with the "pop-up" pistons have a little metal taken out to allow clearance for the raised part of the piston, other wise it will hit in the edge of the chamber around the spark plug. May be other chnges, but that is the only one I know of. Doubt you could tell much by looking at a picture, it is a small change.

The cylinder head changed in 1975, about 7 years after the pop-up pistons were introduced. One has nothing to do with the other. That said, it is always wise to check your engine at assembly to see if the piston hits the head. Heads get re-surfaced, blocks get decked, and castings vary a little. Some have had to do a little grinding- but I would say the majority are OK. If you have no problem with a head gasket, you are good to go. If you do, a little modeling clay will show you where you need to do a little work and if it comes to that, an old head gasket is handy to have. I have some comparison pictures but the later heads are very rare items on Cubs starting at ENGINE number 312390 including Numbered Series Lo-Boys.

Are you saying that that PN head was only used from Eng SN 312390 starting in '75?The 355 691 R2 head was being used on 154 Lowboys since I assume they rolled out in '68. Dome top pistons were also being used since '68 in all model Cubs starting at Eng SN 264568.

From looking at the parts breakdowns, the F-Cubs switched to this same head sometime between '70 and '74, although I can't seem to find an exact SN. I don't see anything in the TC-37F that indicates anything changed with the head in '75.